Discipline: animal health; Key words: liver fluke, L. truncatula, Fasciola parasite, open water, cercaria.
Some interesting results have been reported by Dr Jan van Wyk, who is responsible for the Milk SA project: Fasciola hepatica: Impact on Dairy Production and Sustainable Management on Selected Farms in South Africa. The report is from the fourth quarter of 2018.
The seasonal cycling of the L. truncatula intermediate snail host of the Fasciola parasite follows much the same pattern between the farms in the study, particularly as regards periods that are less conducive to the propagation of the snails. However, there is much variation between the farms, such as in time and number of peaks and variation in the time span of general peaking in numbers of snails recovered. And this is to be expected, since open water is essential for the parasite to be able to infect the snail intermediate hosts, while the snail intermediate host can survive well in wet mud, in the absence of open water. Hence it is to be expected that the peaking of the parasite will not necessarily follow the peaking of the L. truncatula very closely. Another factor is that the parasite has a depressive effect on the health of its intermediate host, which is not well adapted to the higher temperatures in midsummer. This implies that relatively high proportions of the infected snails would not survive long enough under such conditions to allow for completion of the development of the parasite in them. This is something that farmers can take note of.
As mentioned, both the propagation of the snail intermediate host and the transmission of the parasite to the snail are absolutely dependent on open water for transmission of the parasite. In addition, the parasite eggs need to have been deposited in the vicinity of the snails long enough before the open water is present, for the eggs to have “matured” by the time they are required in the open water, so that the eggs are ready to hatch soon after they have become submerged in such water. This train of events needs to be set in motion by periodic grazing of infected cattle on such marshy spots, in this way to allow the eggs to be disseminated to the spots and mature in depressions conducive to accumulaton of open water. Once mature, the eggs can hatch soon after they have entered the water, hatch and successfully infect the snails, in this way to lead to infection of the cattle. In other words, the young parasitic stage of Fasciola, the miracidium, needs to be fully developed to the level of being able to hatch soon after submersion, then swim in the water to find the snail intermediate host, penetrate, and develop to the next stage, the cercaria. Thereupon, once this next state has developed, it similarly requires open water to leave the snail, swim and encyst on herbage of other solid material in their surroundings to the so-called metacercarial stage that is infective to their final hosts, for instance cattle and sheep. This information indicates that dairy farmers should manage their open water areas carefully.